FIELD OF THE INVENTION
The invention lies in the field of fluid dynamics. In particular, the invention pertains to water craft propulsion propellers and to stationary fluid propellers.
The aerodynamic principle utilized in propellers is the effect of the dynamic pressure of the fluid to be propelled on the propeller blade. The resultant dynamic pressure is the sum of all partial pressures acting on the various surfaces of the blades. The dynamic pressure is proportional to the relative speed between the fluid and the propeller blade. Resistance and vortice formation acting on poor fluid-dynamic shapes translates into drag, which is defined as the force counteracting the forward thrust force or torque of the propeller blade. A certain amount of drag cannot be avoided. However, the drag force can be minimized by the proper design of the shape of the blades and its maximization in terms of the useful translational speeds. The object is to minimize drag resistance and to maximize thrust force, i.e., to optimize the thrust-to-drag ratio.
The principles described herein are equally applicable to thrust propellers that are used to propel vehicles through water and pump propellers that are stationary relative to the ground and that are used to pump a flow of water (or other, similarly viscous fluids).
Low speed, low power propellers are relatively simple. The thrust is obtained by a vertical force component acting perpendicularly to a movement of the blades. The thrust is typically parallel to the axis of rotation. A thin plate with a narrow attack surface and a slight backward curve (camber) usually provides a sufficient amount of thrust. In other words, the pitch angle of the forward-most portion of the plate is approximately zero relative to the plane defined by the propeller sweep and the blade has a backward curve by a few degrees. With the relatively low speeds and large blade surfaces of the simple pump propellers, the slightly curved shape of the blade is generally acceptable. As the blade speed is increased, however, the thrust-to-drag ratio very quickly deteriorates, because the resultant turbulent flow, i.e., the vortices or eddies, at the trailing edge of the blade lead to cavitation and super-cavitation at the propeller. Cavitation is thereby defined as the formation of air bubbles and the concurrently severe loss of thrust.
Propeller inefficiency is also affected by micro-friction between the exposed surfaces and the innermost layer (flow sheet) of the fluid impinging and being deflected by the surfaces. This invention, however, is primarily concerned with improving the macro-structure and the thrust-to-drag ratio of watercraft and pump propeller blades.
Similarly to aircraft wing design, where most of the lift on a wing is due to the vacuum effect above the wing (the negative pressure compensates for the fluid compression forward of and below the wing), the "shaded" surface of the propeller blade is important as well. The typical ratio in wings is that approximately two-thirds of the lift originates from the upper vacuum effect and one-third is due to the compression below the wing. This recognition, in the early days of wing design, resulted in the development of the airfoil. The airfoil shape at first glance appears counter-intuitive. The airfoil has a thickened forward section which tapers to a very thin tip structure at the trailing edge. Nevertheless, the basic airfoil shape was also adopted for propeller blades.
While the relatively "thick" layout of the airfoils cannot be adopted in water (this is due to the fact that water is much denser and heavier than air), the principle according to which the rear surface of the propeller (the surface which is in fact facing forward in the direction of movement, i.e. opposite the flow direction) is still very important in the proper layout of the water propeller blade.
As noted, the principles concerning vortice creation and drag in wing designs are similarly applicable to propellers and rotor blades in water. There, the eddie formation principles applicable to the relatively thin fluid air find their equivalents in the denser fluid water with the formation of eddie current vortices, cavitation, and super-cavitation.